Grain oriented electrical steel sheet

ABSTRACT

An electrical steel sheet contains, as components, by mass %, 0.005% or less of C, 1.0% to 8.0% of Si, and 0.005% to 1.0% of Mn; one or more selected from Nb, Ta, V, and Zr such that a total content thereof is 10 to 50 ppm; and the balance being Fe and unavoidable impurities, wherein at least 10% of the content of Nb, Ta, V, and Zr is in the form of precipitates; the precipitates have an average diameter (equivalent circle diameter) of 0.02 to 3 μm; and secondary recrystallized grains of the steel sheet have an average grain size of 5 mm or more.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2010/063343, with an international filing date of Jul. 30, 2010 (WO 2011/013858 A1, published Feb. 3, 2011), which is based on Japanese Patent Application No. 2009-179494, filed Jul. 31, 2009, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a grain oriented electrical steel sheet suitably used as, for example, an iron-core material for transformers and, in particular, reduces degradation of magnetic characteristics in the case of the sheet being sheared.

BACKGROUND

Electrical steel sheets are a material widely used for iron cores of various transformers, motors, and the like. In particular, among them, electrical steel sheets that are referred to as grain oriented electrical steel sheets have crystal grains that are highly oriented in {110}<001>, which is referred to as the Goss orientation.

In the production of such grain oriented electrical steel sheets, a technique of causing secondary recrystallization of crystal grains having the Goss orientation to occur during final annealing with precipitates referred to as an inhibitor is generally used.

For example, Japanese Examined Patent Application Publication No. 40-15644 discloses a method of making Al and S serving as inhibitor-forming elements be present in predetermined amounts, that is, a method of using AlN and MnS as inhibitors. Japanese Examined Patent Application Publication No. 51-13469 discloses a method of making at least one of S and Se be present in a predetermined amount, that is, a method of using MnS or MnSe as an inhibitor. These methods are industrially used. Furthermore, as proposed in Japanese Unexamined Patent Application Publication No. 2000-129356, a technique of developing Goss oriented grains by the action of secondary recrystallization even in steel sheets having no inhibitor-forming elements has been recently presented.

In the technique described in JP '356, by minimizing impurities such as inhibitor-forming elements, the grain boundary misorientation dependency of grain boundary energy of grain boundary in the occurrence of primary recrystallization is elicited so that Goss oriented grains are developed by secondary recrystallization without inhibitors.

Since this method does not require inhibitor-forming elements, the necessity of the step of purifying to remove inhibitor-forming elements is eliminated. In addition, it is not necessary to perform purification annealing at a high temperature and a step of finely dispersing inhibitor-forming elements in steel is no longer necessary. Hence, slab reheating at a high temperature that was indispensable for the fine dispersion is also no longer necessary. Thus, the method is highly advantageous in terms of steps, cost, and maintenance of equipment and the like.

Among various characteristics of grain oriented electrical steel sheets, an iron loss characteristic directly relates to energy loss of products and is considered to be the most important characteristic. To improve the iron loss characteristic, it is believed that a value represented by W_(17/50) (energy loss at an excitation magnetic flux density of 1.7 T and an excitation frequency of 50 Hz) should be decreased.

In transformers for which grain oriented electrical steel sheets are used, the iron loss characteristic is also considered as an important characteristic. Even after transformers are produced, the transformers that are used need to be periodically measured in terms of iron loss characteristic for the purpose of controlling the iron loss characteristic.

In general, electrical steel sheet products have the shape of a sheet and are cut to have a predetermined size in the production of transformers. This cutting is generally performed by shearing (also referred to as slit processing) in which two blades vertically press against each other (the blades finally slide over each other) as in a pair of scissors.

In the thus-sheared steel sheets, the processed surfaces are formed by tearing due to a shearing force and a large amount of strain is introduced into the steel sheets. Accordingly, degradation of magnetic characteristics due to the introduced strain tends to occur in sheared electrical steel sheets, which is problematic.

As a method of reducing degradation of magnetic characteristics due to shearing, stress relief annealing of annealing at 700° C. to 900° C. for several hours may be performed after shearing. However, stress relief annealing is performed only for small transformers having a size (length) of 500 mm or less and it cannot be performed for, for example, iron cores of large transformers having a size of several meters.

Accordingly, a technique has been demanded that can reduce degradation of magnetic characteristics due to shearing in electrical steel sheets for large transformers having a size of several meters.

SUMMARY

We provide:

-   -   1. A grain oriented electrical steel sheet characterized by         comprising, by mass %, 0.005% or less of C, 1.0% to 8.0% of Si,         and 0.005% to 1.0% of Mn; one or more selected from Nb, Ta, V,         and Zr such that a total content thereof is 10 to 50 ppm; and         the balance being Fe and unavoidable impurities, wherein at         least 10% of the content of Nb, Ta, V, and Zr is in the form of         precipitates; the precipitates have an average diameter         (equivalent circle diameter) of 0.02 to 3 μm; and secondary         recrystallized grains of the steel sheet have an average grain         size of 5 mm or more.     -   2. The grain oriented electrical steel sheet according to 1         above, characterized by further comprising at least one selected         from, by mass %, 0.010% to 1.50% of Ni, 0.01% to 0.50% of Cr,         0.01% to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to 0.50% of         Sn, 0.005% to 0.50% of Sb, 0.005% to 0.50% of Bi, and 0.005% to         0.100% of Mo.     -   3. The grain oriented electrical steel sheet according to 1 or 2         above, characterized in that a groove is formed in a surface of         the steel sheet, the groove having a shape of a solid line or a         broken line, a width of 50 to 1,000 μm, and a depth of 10 to 50         μm, and extending at an angle of 15° or less with respect to a         direction perpendicular to a rolling direction of the steel         sheet.     -   4. A method for producing an iron core, characterized by         shearing the grain oriented electrical steel sheet according to         any one of 1 to 3 above to provide sheets and subsequently         stacking the sheets without subjecting the sheets to stress         relief annealing.

Degradation of magnetic characteristics of grain oriented electrical steel sheets due to shearing can thus be effectively suppressed and iron cores having less energy loss can be produced for transformers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relationship between Nb content in steel (abscissa axis: ppm) and the amount of degradation of iron loss due to shearing (ΔW) (ordinate axis: W/kg).

FIG. 2 illustrates the relationship between the crystal grain size of secondary recrystallized grains (abscissa axis: mm) and the amount of degradation of iron loss due to shearing (ΔW) (ordinate axis: W/kg).

DETAILED DESCRIPTION

Hereinafter, our steel sheets and methods will be specifically described.

The reasons why the component composition of a steel sheet is limited to the above-described ranges will be first described. Note that “%” and “ppm” for components of a steel sheet respectively represent mass % and mass ppm, unless otherwise stated.

C: 0.005% or less

C is an element that unavoidably enters steel. Since C causes degradation of magnetic characteristics by magnetic aging, the C content is desirably minimized. However, it is difficult to completely remove C and a C content of 0.005% or less is allowable in view of production cost, preferably 0.002% or less. There is no reason for particularly defining the lower limit of the C content. The C content is industrially more than zero.

Si: 1.0% to 8.0%

Si is an element necessary to increase the resistivity of steel and achieve improvement in terms of iron loss in final product sheets. When the Si content is less than 1.0%, such an advantage is not sufficiently provided. When the Si content is more than 8.0%, the saturation flux density of a steel sheet considerably decreases. Accordingly, the Si content is limited to a range of 1.0% to 8.0%. The lower limit of the Si content is preferably 3.0%. The upper limit of the Si content is preferably 3.5%.

Mn: 0.005% to 1.0%

Mn is an element necessary to enhance formability in hot rolling. When the Mn content is less than 0.005%, the effect of enhancing workability is not sufficiently provided. When the Mn content is more than 1.0%, secondary recrystallization becomes unstable and magnetic characteristics are degraded. Accordingly, the Mn content is limited to a range of 0.005% to 1.0%. The lower limit of the Mn content is preferably 0.02%. The upper limit of the Mn content is preferably 0.20%.

It is necessary to make one or more selected from Nb, Ta, V, and Zr (hereafter, referred to as “Nb or the like”) be contained as precipitate-forming elements such that the total content thereof is 10 to 50 ppm. This is because, when the total content of Nb or the like is less than 10 ppm, precipitates that improve iron loss, which are a main feature, are not sufficiently generated. When the total content of Nb or the like is more than 50 ppm, the iron loss characteristic of a material itself is degraded as described above. Thus, the upper limit of the total content is defined as 50 ppm. The total content is preferably in the range of 10 to 30 ppm.

It is necessary that the precipitates of Nb or the like are present in a percentage of 10% or more and the precipitates have an average diameter (equivalent circle diameter) of 0.02 to 3 μm. When the average diameter is less than 0.02 μm, the precipitates are too small and stress is less likely to be concentrated. When the average diameter is more than 3 μm, the frequency of the presence (number) of the precipitates becomes small and the number of portions where stress is concentrated becomes small. The precipitates preferably have an average diameter of 0.05 to 3 μm. The lower limit is more preferably 0.12 μm, still more preferably 0.33 μm. The upper limit is more preferably 1.2 μm, still more preferably 0.78 μm.

The precipitation percentage of the precipitates of Nb or the like is preferably 20% or more, more preferably 31% or more, still more preferably 48% or more. It is not necessary to define the upper limit and a precipitation percentage of 100% does not cause problems.

The average diameter of the precipitates of Nb or the like is preferably determined in the following manner: a section of an obtained sample is observed with a scanning electron microscope; micrographs of about 10 fields of view are taken at a magnification of about 10,000; the micrographs are subjected to image analysis and the average of equivalent circle diameters is determined. The percentage of precipitates (precipitation percentage) is preferably measured in accordance with the method described in Experiment 1 below. When a steel sheet contains two or more elements as Nb or the like, the total content (mass %) of Nb or the like in precipitates should be divided by the total content (mass %) of Nb or the like in the steel sheet.

As a precipitate-forming element, one or more selected from Nb, V, and Zr are preferred because they are less likely to form defects in steel sheets during hot rolling. In particular, Nb is preferred because defects during hot rolling can be reduced. In such cases, the essential range is also 10 to 50 ppm and the preferred range is also 10 to 30 ppm and a preferred diameter of precipitates and a preferred precipitation percentage are the same as those described above.

To adjust the diameter and the precipitation percentage of precipitates of Nb or the like, it is effective to control, in purification annealing, the maximum steel sheet temperature and a cooling rate in the subsequent cooling from 900° C. to 500° C. This is because such precipitates can be controlled in terms of diameter and precipitation percentage by performing purification annealing at a high temperature to dissolve the precipitates and performing cooling to cause reprecipitation.

In such a phenomenon, as in general precipitation phenomena, a high cooling rate results in a low amount of precipitates (a portion remains in the form of a solid solution) and a small diameter of precipitates and, in contrast, a low cooling rate tends to result in the reverse state.

As described above, to exhibit the effect of decreasing ΔW by addition of a precipitate-forming element, it is necessary that the average grain size of secondary recrystallized grains of a material is 5 mm or more. Although such a grain size is a general grain size in electrical steel sheets for large transformers having a size of several meters, regardless of such a sheet size, by controlling a temperature increase rate and an atmosphere in secondary recrystallization, the average grain size can be controlled to be 5 mm or more. The average grain size of secondary recrystallized grains is preferably determined by the method described in Experiment 2 below.

Note that a method of decreasing ΔW by making the average grain size of secondary recrystallized grains be less than 5 mm is not preferred because the absolute values of iron loss and magnetic flux density become poor.

The basic component composition and the like have been described so far.

If necessary, elements described below may appropriately be contained.

Ni: 0.010% to 1.50%

To enhance magnetic characteristics, Ni may be added. In such a case, when the amount of Ni added is less than 0.010%, magnetic characteristics are not sufficiently enhanced. When the amount of Ni added is more than 1.50%, secondary recrystallization becomes unstable and magnetic characteristics may be degraded. Accordingly, the Ni content is preferably made in the range of 0.010% to 1.50%.

Cr: 0.01% to 0.50%, Cu: 0.01% to 0.50%, P: 0.005% to 0.50%

To decrease iron loss, at least one of Cr, Cu, and P may be added.

However, when the amounts of the elements added are less than the lower limits, the effect of decreasing iron loss is not sufficiently provided. When the amounts of the elements added are more than the upper limits, development of secondary recrystallized grains is suppressed, resulting in an unintended increase in iron loss. Accordingly, the contents of the elements are preferably in the ranges described above, respectively.

Sn: 0.005% to 0.50%, Sb: 0.005% to 0.50%, Bi: 0.005% to 0.50%, Mo: 0.005% to 0.100%

To increase magnetic flux density, at least one of Sn, Sb, Bi, and Mo may be added.

However, when the amounts of the elements added are less than the lower limits, the effect of enhancing the magnetic characteristic is not sufficiently provided. When the amounts of the elements added are more than the upper limits, development of secondary recrystallized grains is suppressed, resulting in degradation of the magnetic characteristic. Accordingly, the contents of the elements are preferably in the ranges described above, respectively.

In summary, an electrical steel sheet may further contain at least one selected from 0.010% to 1.50% of Ni, 0.01% to 0.50% of Cr, 0.01% to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to 0.50% of Sn, 0.005% to 0.50% of Sb, 0.005% to 0.50% of Bi, and 0.005% to 0.100% of Mo. Further, as for a subset constituted by elements freely selected from the group of these elements, at least one selected from elements (group) constituting the subset may be made to be contained.

In addition, if necessary, at least one combination of inhibitor-forming elements (for example, AlN-forming elements: Al and N, MnS-forming elements: Mn and S, MnSe-forming elements: Mn and Se, and TiN-forming elements: Ti and N) in a necessary amount (publicly known) can be contained.

The balance is Fe and normal unavoidable impurities. Examples of the unavoidable impurities include P, S, O, Al, N, Ti, Ca, and B (when Al and the like are not added as inhibitor-forming elements, they are impurities).

Grooves are preferably formed in a surface of a steel sheet, the grooves having the shape of a solid line or broken lines, a width of 50 to 1,000 μm, and a depth of 10 to 50 μm, and extending in a direction to intersect at an angle of 15° or less in a direction perpendicular to the rolling direction. The formation of such grooves provides the magnetic domain refining effect, resulting in a further decrease in iron loss. The space between the grooves (pitch) is preferably about 2 to 7 mm. When grooves extend at an angle of 0° with respect to a direction perpendicular to a rolling direction, in a strict sense, the grooves do not intersect the direction perpendicular to the rolling direction. However, such a case is also referred to as an “intersection.” In summary, grooves should be formed at an angle of 15° or less with respect to a direction perpendicular to a rolling direction.

As a result of the formation of such grooves, the iron loss of an electrical steel sheet decreases by about 0.17 W/kg. Such an advantage was found to be achieved regardless of selection of an element from Nb, Ta, V, and Zr.

Hereinafter, a preferred method for producing a grain oriented electrical steel sheet will be described. As main steps in this production method, production steps for a standard grain oriented electrical steel sheet can be used. Specifically, a series of steps can be used in which slabs produced from a molten steel adjusted to have a predetermined component composition are hot-rolled; the resultant hot-rolled sheets are optionally subjected to hot-rolled sheet annealing and then subjected to a single cold-rolling step or two or more cold-rolling steps that include an intermediate annealing therebetween to have a final sheet thickness. The steel sheets are subsequently subjected to recrystallization annealing, then to purification annealing, and optionally to flattening annealing and the steel sheets are then coated.

In the case of adjusting the component composition of molten steel, when the amount of C added is more than 0.10%, it is difficult to decrease in subsequent steps the C content to 50 ppm (0.005%) or less, which does not cause magnetic aging. Accordingly, the amount of C added in molten steel is preferably 0.10% or less.

The Si content may be adjusted to be 1.0% to 8.0%, which is the finally required content, in the adjustment of the component composition of molten steel. When a method of increasing Si content by siliconization or the like is employed in a step after the production of slabs, the amount of Si added to molten steel may be less than the finally required content.

It is difficult to add or remove Nb, Ta, V, and Zr, which are essential components, during steps after the molten steel state. Accordingly, it is most desirable that a required amount of such a component be added in the adjustment of the component composition of molten steel.

From molten steel containing the components described above, slabs may be produced by a standard ingot making process or a standard continuous casting process, or otherwise thin cast slabs having a thickness of 100 mm or less may be produced by direct casting process. Although slabs are heated and hot-rolled in a standard manner, slabs after being cast may be instead directly hot-rolled without being heated. In the case of thin cast slabs, it may be hot-rolled or jump straight to next steps without being hot-rolled.

The heating temperature of slabs to be hot-rolled in a component system containing an inhibitor-forming element is normally a high temperature of about 1,400° C. In contrast, the heating temperature in a component system without inhibitor-forming elements is normally a low temperature of 1,250° C. or less, which is advantageous in terms of cost.

If necessary, hot-rolled sheet annealing is then performed. To achieve good magnetic properties, the temperature of the hot-rolled sheet annealing is preferably 800° C. or more and 1,150° C. or less. This is because, when the temperature of the hot-rolled sheet annealing is less than 800° C., a band texture due to hot rolling remains and it becomes difficult to achieve a primary recrystallization texture having uniformly-sized grains. Accordingly, the hot-rolled sheet annealing provides a relatively limited effect of promoting development of secondary recrystallized grains. When the temperature of the hot-rolled sheet annealing is more than 1,150° C., crystal grains after the hot-rolled sheet annealing become coarse. Accordingly, also in this case, it becomes difficult to achieve a primary recrystallization texture having uniformly-sized grains.

After the hot-rolled sheet annealing, one or more cold-rolling steps optionally including a process an intermediate annealing therebetween are performed and recrystallization annealing is then performed. To further enhance magnetic characteristics, it is effective to perform cold rolling in a temperature range of 100° C. to 300° C. and/or to perform one or more aging treatments in a range of 100° C. to 300° C. during the cold rolling process. In the case of performing recrystallization annealing, when decaburization is necessary, a wet atmosphere is employed in the recrystallization annealing. However, when decaburization is not necessary, the recrystallization annealing may be performed in a dry atmosphere. After the recrystallization annealing, a technique of increasing Si content by siliconization may be further employed.

When iron loss is considered as an important factor and a forsterite coating is subsequently formed, the sheets are coated with an annealing separator mainly containing MgO and then subjected to final annealing (purification annealing) to develop a secondary recrystallization texture and to form a forsterite coating.

When a blanking property is considered as an important factor and a forsterite coating is not intentionally formed, an annealing separator is not applied or, even when an annealing separator is applied, silica, alumina, or the like should be used instead of MgO forming a forsterite coating.

When such an annealing separator is applied, for example, electrostatic coating without involving water content is effectively performed. A heat-resistant inorganic material sheet (silica, alumina, or mica) may be used.

The final annealing is sufficiently performed at a temperature allowing for secondary recrystallization, and desirably at 800° C. or more. An annealing condition under which secondary recrystallization is completed is desirable and it is generally desirable that the sheets be held at a temperature of 800° C. or more for 20 or more hours. When a blanking property is considered as an important factor and a forsterite coating is not formed since secondary recrystallization only needs to be completed, the holding temperature is desirably about 850° C. to 950° C. and the final annealing may be finished with this holding treatment. When iron loss is considered as an important factor or the noise of a transformer is to be reduced, and a forsterite coating is formed, the temperature is advantageously increased to about 1,200° C.

In cooling in such a high-temperature annealing, the cooling is desirably performed at a rate of 5° C./hr to 100° C./hr at least in a temperature range of 900° C. to 500° C. When cooling is performed from a holding temperature less than 900° C., the cooling is desirably performed at a rate of 5° C./hr to 100° C./hr in a temperature range of the holding temperature to 500° C. This is because, when the cooling rate is more than 100° C./hr in such a temperature range, there may be cases where precipitates become excessively fine or precipitation from a solid solution does not occur. When the cooling rate is less than 5° C./hr, there may be cases where the diameter of precipitates becomes excessively large, or the cooling time becomes excessively long resulting in, for example, degradation of productivity. The lower limit of the cooling rate is more preferably 7.8° C./hr. The upper limit of the cooling rate is more preferably 30° C./hr. In view of achieving results with stability, the upper limit of the cooling rate is still more preferably 14° C./hr.

After the final annealing, to remove an annealing separator adhering, it is effective to perform cleaning with water, brushing, and/or pickling. After that, it is effective to subject the sheets to flattening annealing to correct the shape thereof for the purpose of decreasing iron loss.

When the steel sheets are laminated and used to achieve improvement in terms of iron loss, it is effective to form insulation coatings on the surfaces of the steel sheets before or after the flattening annealing. To decrease iron loss, coatings that can impart tension to steel sheets are desirable. When a method of coating the surfaces of a steel sheet with an inorganic substance by a tension coating application method with a binder, a physical vapor deposition method, a chemical vapor deposition method, or the like is employed, the coating films exhibit high adhesion and iron loss is considerably decreased, which is particularly desirable.

To decrease iron loss, a magnetic domain refining treatment is desirably performed. An example of this treatment is, as generally performed, a method of forming grooves in final product sheets or linearly introducing thermal strain or impact strain with laser or plasma into final product sheets, or a method of forming grooves in intermediate products having a final sheet thickness such as cold-rolled sheets.

As a preferred method for producing an iron core using steel sheets, for example, there is provided the method including shearing steel sheets and laminating the sheets without subjecting them to stress relief annealing. At this time, degradation of iron loss of the steel sheet due to the shearing can be suppressed to 0.1 W/kg or less (preferably, 0.041 W/kg or less). The production method is particularly advantageous for producing large iron cores, for example, in the cases where a steel sheet is sheared into sheets having a longest side more than 500 mm. Matters including the number of steel sheets stacked, the size and shape of steel sheets obtained by the shearing, the presence or absence of the grooves, the size of the grooves, the presence or absence of coating, and the type of coating may be appropriately determined on the basis of ordinary knowledge.

We found that a small content of an element such as Nb can considerably reduce degradation of iron loss due to shearing.

Hereinafter, experiments will be described.

Experiment 1

Grain oriented electrical steel sheets containing, by mass %, 3.30% to 3.34% of Si, 0.06% to 0.07% of Mn, 0.025% to 0.028% of Sb, and 0.03% to 0.04% of Cr; Nb added in various amounts of 4 ppm (on the level of unavoidable impurities), 22 ppm, 48 ppm, 65 ppm, 90 ppm, and 210 ppm; and the balance being Fe and unavoidable impurities were produced by a standard production method having recrystallization annealing (primary recrystallization annealing) and final annealing (purification annealing). In the final annealing (purification annealing), the steel sheets were heated at the maximum steel sheet temperature of 1,200° C. to dissolve the precipitate-forming element (Nb) therein and then cooled at an average cooling rate of 20° C./hr from 900° C. to 500° C. and cooled to room temperature.

The thus-obtain grain oriented electrical steel sheets were cut into so-called “Epstein” specimens having a size of 30 mm×280 mm. At this time, two types of specimens were prepared by a process of slowly cutting the steel sheets with a wire cutter such that strain was not caused in the steel and by a general cutting process for grain oriented electrical steel sheets in which the steel sheets were cut with a shearing machine employing an upper blade and a lower blade as described above. The resultant samples were measured in terms of iron loss in accordance with a method described in JIS C 2550.

FIG. 1 shows the results of a study about the relationship between ΔW (ordinate axis: W/kg) and Nb content in steel (abscissa axis: mass ppm), ΔW (hereafter, same definition) being determined by subtracting the iron loss value of a sample obtained by cutting with the wire cutter from the iron loss value of a sample obtained by cutting with the shearing machine.

In the case of cutting with the shearing machine, as described above, strain remained in the steel sheets and the iron loss of the steel sheets was degraded. In contrast, cutting with the wire cutter took a long time, but the steel sheets were cut substantially without causing strain to remain in the steel sheets.

Accordingly, it is believed that ΔW in the figure substantially represents an iron loss amount equivalent to degradation due to remaining strain. FIG. 1 thus shows that the presence of Nb results in reduction of degradation of the iron loss amount due to shearing.

The reason why degradation of the iron loss of the Nb-containing samples was reduced as described above is not necessarily clear. We believe the following:

-   -   An analysis of the microstructure of the Nb-containing material         used in the experiment revealed that Nb forms precipitates and         is dispersed in the steel. Small precipitates had a diameter of         about 0.02 μm and large precipitates had a diameter of about 3         μm. Since normal grain oriented electrical steel sheets do not         substantially have such precipitates in steel, we believe that         the presence of the precipitates probably contributed to         reduction of degradation of iron loss due to shearing.     -   Degradation of iron loss due to shearing is caused by         accumulation of strain in portions having been subjected to         shearing. Accumulation of strain is a phenomenon where iron         atoms regularly arranged in iron crystal grains are subjected to         an external stress or the like and the arrangement of iron atoms         is distorted or becomes irregular.

Consider a case where the above-described precipitates are present in such regularly arranged iron atoms. When a stress due to shearing or the like is applied to a portion containing the precipitates to cut the portion, the stress is concentrated on the periphery of the precipitates and cracking is probably generated before the arrangement of iron atoms is distorted. When it is considered that such a mechanism relieves the accumulation of strain, the above-described phenomenon can be explained.

Although Nb contained in a steel sheet is in two states of forming a solid solution and forming precipitates, as described above, it is probably important that Nb forms precipitates. Thus, the sample containing 22 ppm of Nb was measured in terms of Nb precipitation percentage (percentage of Nb content in precipitates with respect to the total Nb content).

To determine the Nb precipitation (i.e., Nb of Nb precipitates) percentage, the total Nb content (content in a steel sheet: mass %) needs to be first determined. The total Nb content can be determined by inductively-coupled plasma optical emission spectrometry (ICP optical emission spectrometry) described in JIS G 1237. Note that the contents of Ta, V and Zr can be respectively determined by methods described in JIS G 1236, JIS G 1221 and JIS G 1232.

The Nb content in precipitates (content in a steel sheet: mass %) can be determined by melting a steel sheet by electrolysis to capture precipitates only (by filtration), measuring the weight of Nb in the precipitates, and calculating from a decrease in the weight of the steel sheet due to electrolysis and the weight of Nb in the precipitates.

Specifically, the quantitative value of Nb content in precipitates is determined in the following manner.

A product sheet is first cur to a size of 50 mm×20 mm and immersed for 2 minutes in a 10% aqueous solution of HCl heated at 85° C. to remove the coating and film of the product. After that, the weight of the product sheet is measured. The product sheet is electrolyzed with a commercially available electrolytic solution (10% AA solution: 10% acetylacetone-1% tetramethylammonium chloride-methanol) such that about 1 g of the product sheet is electrolyzed. To remove precipitates adhering to the surfaces of the product sheet electrolyzed, the product sheet is immersed in an ethanol solution and subjected to ultrasonic waves.

This ethanol solution and the electrolytic solution used in the electrolysis, which contain precipitates, are filtrated through a 0.1 μm-mesh filter paper (allowing capture of minimum precipitates having a size on the order of nanometers) to capture the precipitates. After filtration, the precipitates collected by the filtration are placed together with the filter paper in a platinum crucible, heated at 700° C. for an hour, mixed with Na₂B₄O₇ and NaCO₃, and heated at 900° C. for 15 minutes. The resultant substance is cooled and then heated at 1,000° C. for 15 minutes.

After cooling, the substance in the crucible coagulates. The crucible containing the substance is placed into a 25% aqueous solution of HCl and the solution containing the crucible is heated at 90° C. for 30 minutes to melt the entirety of the substance. The resultant solution is analyzed by ICP optical emission spectrometry described in JIS G 1237 to determine the weight of Nb in the precipitates.

The weight of Nb is divided by a decrease in the weight of the product sheet (steel sheet) due to electrolysis to determine the Nb content (mass %) in the precipitates.

The thus-determined Nb content (mass %) in the precipitates is divided by the total Nb content (mass %) to determine the Nb precipitation percentage.

The Nb precipitation percentage in the sample was 65%. We further performed studies and have found that precipitation of at least 10% of the total Nb content is necessary to provide desired advantages.

In view of the above-described mechanism, the more the amount of a precipitate-forming element such as Nb remaining in steel, the better the ΔW characteristic seems to become. However, precipitates also degrade the iron loss characteristic of a material itself to be processed. Accordingly, the amount of precipitates is preferably small within a range in which degradation of iron loss due to shearing is small. In Experiment 1, in materials having a Nb content of 65 ppm or more, the iron loss of the materials themselves degraded. Hence, the content needs to be suppressed to 50 ppm or less.

Next, influence of the crystal grain size of secondary recrystallized grains on ΔW was studied. This is because we believe that the presence of a large number of grain boundaries also probably relieves the accumulation of strain due to shearing. Accordingly, when the crystal grain size is small and a large number of grain boundaries are present, there may be cases where degradation of iron loss due to shearing is naturally small and the above-described mechanism of relieving accumulation of strain due to precipitates does not provide advantages.

Experiment 2

Steel slabs containing, by mass %, 0.035% of C, 3.31% of Si, 0.13% of Mn, 0.039% of Sb, 0.05% of Cr, and 0.012% of P; 42 ppm of N and 31 ppm of S; and the balance being Fe and unavoidable impurities were produced by continuous casting, subjected to slab reheating at 1,250° C., then hot-rolled to provide hot-rolled sheets having a thickness of 2.7 mm. The hot-rolled sheets were subsequently annealed at 1,000° C. for 15 seconds and then cold-rolled to provide sheets having a thickness of 0.30 mm.

The sheets were subjected to recrystallization annealing in a 50% N₂-50% H₂ wet atmosphere (decarburization atmosphere) under soaking conditions in a temperature range of 800° C. to 880° C. for 60 seconds. The sheets were then coated with an annealing separator mainly containing MgO and subsequently subjected to purification annealing by being retained in a temperature range of 1,050° C. to 1,230° C. for 10 hours.

The temperatures in the recrystallization annealing and the purification annealing were varied to vary crystal grain size provided by secondary recrystallization caused in the purification annealing.

Flattening annealing that also allows for formation of a tension coating mainly containing magnesium phosphate and boric acid was then performed at 900° C. for 15 seconds. The resultant sheets were cut to have the size of Epstein specimens (30 mm×280 mm). At this time, as in Experiment 1, cutting with a wire cutter and cutting with a shearing machine were performed. The resultant samples were measured in terms of iron loss in accordance with a method described in JIS C 2550.

After that, the steel substrates were exposed by pickling and the crystal grain size of secondary recrystallized grains was measured. For each condition, the crystal grain size was determined by measuring the grain sizes of four Epstein specimens and averaging the measured grain sizes. Analysis of the components of the steel substrates revealed 0.0018% of C, 3.30% of Si, 0.13% of Mn, 0.039% of Sb, 0.05% of Cr, and 0.011% of P, and the contents of the other elements were less than the detection limits. The relationship between ΔW (ordinate axis: W/kg) determined in the above-described manner and crystal grain size (abscissa axis: mm) is illustrated in FIG. 2.

In Experiment 2, since precipitate-forming elements such as Nb did not remain, the advantages provided in Experiment 1 were not exhibited. Accordingly, when the average grain size was large, ΔW was large; when the average grain size was small, ΔW was small. Stated another way, the effect of decreasing ΔW by addition of a precipitate-forming element such as Nb is exhibited when the average grain size of secondary recrystallized grains is 5 mm or more.

From the above-described experiments, we found that, by making a final product sheet of a grain oriented electrical steel sheet having a large grain size of secondary recrystallized grains contain 10 to 50 ppm of an element such as Nb and by making at least 10% of the content of the element be present in the form of precipitates, degradation of iron loss due to shearing can be suppressed.

EXAMPLE 1

Steel slabs containing 0.065% of C, 3.25% of Si, 0.13% of Mn, 240 ppm of Al, 70 ppm of N, 36 ppm of S, and 25 ppm of Nb (for No. 7 steel only, 20 ppm of Nb), and the balance being Fe and unavoidable impurities, were produced by continuous casting. The steel slabs were subjected to slab reheating at 1,400° C. and then hot-rolled to sheets to a thickness of 2.4 mm. The sheets were then subjected to hot-rolled sheet annealing at 1,000° C. for 40 seconds, subsequently to cold rolling so as to have a thickness of 1.6 mm, to intermediate annealing at 900° C., and then to cold rolling to sheets so as to have a thickness of 0.23 mm.

The resultant sheets were then subjected to recrystallization annealing in a 60% N₂-40% H₂ wet atmosphere under soaking conditions at 850° C. for 90 seconds, subsequently coated with an annealing separator mainly containing MgO, and subjected to purification annealing at 1,220° C. for 6 hours. In the purification annealing, the cooling rate for a range of 900° C. to 500° C. was controlled as described in Table 1 to thereby vary the diameter of Nb precipitates and Nb precipitation percentage. After that, the sheets were subjected to flattening annealing at 850° C. for 20 seconds.

The obtained samples were cut to a size of 30 mm×280 mm. At this time, the cutting was performed under two conditions: cutting with a wire cutter and cutting with a shearing machine. Magnetic characteristics of obtained samples were measured by the method described in JIS C 2550 and the magnetic characteristics of the samples obtained by the cutting with the wire cutter are described in Table 1.

As for iron losses in terms of the two cutting-process conditions, ΔW determined by subtracting the iron loss of a sample obtained by cutting with the wire cutter from the iron loss of a sample obtained by cutting with the shearing machine is also described in Table 1.

The samples having been subjected to the magnetic measurement were then subjected to pickling to remove coatings and the crystal grain size of secondary recrystallized grains was measured. The results are also described in Table 1 together with the measurement results of the diameter and precipitation percentage of Nb precipitates. After pickling, the component composition of steel sheets of the coating-removed samples was measured. As a result, the component composition confirmed was 0.0016% of C, 3.24% of Si, 0.13% of Mn, and 18 ppm of Nb (for No. 7 steel only, 15 ppm of Nb), which satisfied our requirements.

TABLE 1 Cooling Crystal Precipitate Precipitation rate grain size diameter percentage B₈ W_(17/15) ΔW No. (° C./hr) (mm) (μm) (%) (T) (W/kg) (W/kg) Remark 1 2.2 19.8 4.5  98 1.946 0.811 0.101 Comparative Example 2 5.5 20.1 2.5  94 1.942 0.823 0.061 Example 3 7.8 21.2 0.78 91 1.942 0.826 0.036 Example 4 14.0 20.5 0.35 68 1.938 0.795 0.022 Example 5 30 20.2 0.12 55 1.945 0.825 0.038 Example 6 100 17.4 0.08 34 1.938 0.799 0.041 Example  7* 100 18.5 0.08 14 1.94 0.81 0.055 Example 8 250 22 0.03  7 1.937 0.846 0.113 Comparative Example 9 2400 22.6 0.01  3 1.921 1.108 0.214 Comparative Example *The Nb content was 20 ppm in the slab and 15 ppm in the coating-removed sample.

As described in Table 1, all of our Examples in which the crystal grain size and the diameter and precipitation percentage of Nb precipitates satisfy appropriate ranges have good magnetic characteristics and small ΔW, which shows that degradation of iron loss due to shearing is small.

EXAMPLE 2

Product sheets (sheet thickness: 0.23 mm) of grain oriented electrical steel sheets were provided that contained components described in Table 2 and that were produced by a standard production method in which recrystallization annealing was performed, followed by purification annealing at 1,150° C., and cooling at a cooling rate in the range of 900° C. to 500° C. of 25° C./hr.

The grain oriented electrical steel sheets were cut to a size of 30 mm×280 mm. At this time, the cutting was performed under two conditions: cutting with a wire cutter and cutting with a shearing machine.

The magnetic characteristics of the obtained samples were measured by the method described in JIS C 2550 and the magnetic characteristics of the samples obtained by the cutting with the wire cutter are described in Table 2. In addition, ΔW determined as in EXAMPLE 1 is also described in Table 2.

The samples having been subjected to the magnetic measurement were subjected to pickling to remove coatings and the crystal grain size of secondary recrystallized grains was measured. The results are also described in Table 2 together with the measurement results of the diameter and precipitation percentage of precipitates of Nb or the like. Note that the component compositions of steel sheets in Table 2 are results obtained by measuring the component compositions of coating-removed samples after the pickling.

In addition, the precipitates were measured. As a result, the precipitates had an average diameter of 0.05 to 3.34 μm and a precipitation percentage of 0% to 79%.

TABLE 2 C Si Mn Ni Cr Cu P Sn Sb Bi Mo No. (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Other 7 0.0011 3.11 0.02 — — — 0.005 — — — — Ta: 30 ppm 8 0.0026 3.12 0.08 — 0.03 0.01 — 0.045 — — — Ta: 40 ppm 9 0.0031 2.55 0.32 0.06 0.12 0.03 0.02  — 0.065 — — Ta: 30 ppm 10 0.0020 3.31 0.50 — 0.06 — 0.01  — — 0.015 — Ta: 30 ppm 11 0.0017 3.20 0.09 — — — 0.008 — — — 0.013 Ta: 20 ppm 12 0.0022 3.35 0.09 — 0.05 0.01 0.012 — 0.033 — 0.011 Ta: 80 ppm 13 0.0026 3.13 0.07 — 0.06 — 0.011 0.078 0.023 — — Nb: 40 ppm 14 0.0025 3.30 0.13 — 0.05 — 0.011 — 0.123 — — V: 30 ppm 15 0.0019 3.22 0.12 — 0.07 — 0.013 — 0.056 — — Zr: 20 ppm 16 0.0020 3.25 0.63 — 0.05 — 0.021 — 0.058 — — Nb: 18 ppm, Ta: 20 ppm 17 0.0032 2.98 0.07 — 0.05 — 0.018 — 0.04  — — V: 20 ppm, Zr: 20 ppm, Nb: 9 ppm 18 0.0017 3.01 0.21 — 0.06 — 0.018 — 0.052 — — — 19 0.0028 3.34 0.17 — 0.05 — 0.015 — 0.039 — — Nb: 35 ppm 20 0.0015 3.21 0.06 — — — — — — — — Ta: 30 ppm 21 0.0020 3.25 0.06 — — — — — — — — Nb: 40 ppm 22 0.0020 3.11 0.15 — — — — — — — — V: 30 ppm 23 0.0031 3.35 0.20 — — — — — — — — Zr: 30 ppm Precipitate Precipitation Crystal diameter percentage grain size B₈ W_(17/50) ΔW No. (μm) (%) (mm) (T) (W/kg) (W/kg) Remark 7 0.34 42 12.3 1.943 0.798 0.031 Example 8 0.08 38  7.8 1.955 0.760 0.022 Example 9 0.15 41 16.5 1.942 0.821 0.015 Example 10 0.08 55 10   1.938 0.839 0.038 Example 11 0.52 60 12.1 1.937 0.824 0.021 Example 12 3.34 79 10.5 1.940 1.135 0.026 Comparative Example 13 0.35 50 23.9 1.966 0.759 0.025 Example 14 0.19 43 17.7 1.956 0.780 0.034 Example 15 0.05 31 14.4 1.942 0.803 0.025 Example 16 0.07 32 20.5 1.946 0.845 0.041 Example 17 0.20 40 13.4 1.950 0.847 0.036 Example 18 — 0 22.0 1.953 0.795 0.186 Comparative Example 19 0.51 35  2.3 1.884 1.035 0.023 Comparative Example 20 0.08 40 20.1 1.950 0.791 0.023 Example 21 0.07 42 25.4 1.960 0.777 0.040 Example 22 0.08 36 18.0 1.942 0.801 0.035 Example 23 0.45 51 17.2 1.943 0.809 0.039 Example

As described in Table 2, all of our Examples in which the crystal grain size and the diameter and precipitation percentage of precipitates of Nb or the like satisfy appropriate ranges have good magnetic characteristics and small ΔW, which shows that degradation of iron loss due to shearing is small.

EXAMPLE 3

Steel slabs containing 0.065% of C, 3.25% of Si, 0.13% of Mn, 0.05% of Cr, 240 ppm of Al, 70 ppm of N, 36 ppm of S, 0.013% of P, 0.075% of Sn, 0.036% of Sb, 0.011% of Mo, and 25 ppm of Nb, and the balance being Fe and unavoidable impurities, were produced by continuous casting. The steel slabs were subjected to slab reheating at 1,400° C. and then hot-rolled to sheets to a thickness of 2.4 mm. The sheets were then subjected to hot-rolled sheet annealing at 1,000° C. for 40 seconds, subsequently to cold rolling so as to have a thickness of 1.6 mm, to intermediate annealing in a temperature range of 700° C. to 1,020° C., and then to cold rolling to provide steel sheets having a thickness of 0.23 mm.

Linear grooves having a width of 100 μm and a depth of 25 μm were then formed by local electrolytic etching in the surfaces of the steel sheets to extend at an angle of 10° with respect to a direction perpendicular to the rolling direction at a pitch of 8 mm. The sheets were then subjected to recrystallization annealing in a 60% N₂-40% H₂ wet atmosphere under soaking conditions at 800° C. to 900° C. for 90 seconds. The sheets were then coated with an annealing separator mainly containing MgO and subsequently subjected to purification annealing at 1,220° C. for 6 hours. After that, the sheets were cooled such that they were cooled from 900° C. to 500° C. at a cooling rate of 10° C./hr.

The sheets were then subjected to flattening annealing at 850° C. for 20 seconds. The temperatures of the intermediate annealing and the temperatures of the recrystallization annealing were varied to vary the grain size after secondary recrystallization. The obtained samples were cut into Epstein specimens having a size of 30 mm×280 mm. At this time, the cutting was performed under two conditions: cutting with a wire cutter and cutting with a shearing machine.

The magnetic characteristics of the obtained samples were measured by the method described in JIS C 2550 and the magnetic characteristics of the samples obtained by the cutting with the wire cutter are described in Table 3. In addition, ΔW determined as in EXAMPLE 1 is also described in Table 3.

The samples having been subjected to the magnetic measurement were subjected to pickling to remove coatings and the crystal grain size of secondary recrystallized grains was measured. The results are also described in Table 3 together with the measurement results of the diameter and precipitation percentage of Nb precipitates. After pickling, the component composition of steel sheets of the coating-removed samples was measured. As a result, the component composition confirmed was 0.0016% of C, 3.24% of Si, 0.13% of Mn, 0.05% of Cr, 0.011% of P, 0.074% of Sn, 0.036% of Sb, 0.011% of Mo, and 18 ppm of Nb, which satisfied our requirements.

TABLE 3 Crystal grain Precipitate Precipitation B₈ W_(17/50) ΔW No. size (mm) diameter (μm) percentage (%) (T) (W/kg) (W/kg) Remark 24  3.8 0.56 71 1.855 0.867 0.015 Comparative Example 25  7.5 0.78 65 1.908 0.695 0.024 Example 26 12.6 0.47 52 1.915 0.690 0.026 Example 27 15.0 0.33 48 1.910 0.681 0.033 Example 28 21.9 0.41 60 1.922 0.689 0.034 Example 29 25.2 0.59 72 1.918 0.677 0.030 Example

As described in Table 3, all of our Examples in which the crystal grain size and the diameter and precipitation percentage of Nb precipitates satisfy appropriate ranges have good magnetic characteristics and small ΔW, which shows that degradation of iron loss due to shearing is small.

EXAMPLES 1 to 3 show that grain oriented electrical steel sheets substantially having a ΔW of 0.1 W/kg or less and undergoing little degradation of magnetic characteristics due to shearing can be provided. Accordingly, production of a laminated iron core by shearing a steel sheet without performing stress relief annealing is effective for enhancing the magnetic characteristics of the iron core, in particular, for achieving improvement in terms of iron loss.

In particular, in the steels containing Nb precipitates in EXAMPLES 1 to 3, the diameter (average diameter) of the precipitates is 0.12 μm or more and 1.2 μm or less (preferably 0.78 μm or less; the precipitation percentage is preferably 48% or more) and ΔW is 0.038 W/kg or less. Thus, better characteristics can be achieved. EXAMPLES 1 to 3 and the like show that, to achieve the diameter and amount of precipitates, the cooling rate after final annealing is preferably made 7.8° C./hr to 30° C./hr, more preferably 7.8° C./hr to 14° C./hr.

INDUSTRIAL APPLICABILITY

Degradation of magnetic characteristics of a grain oriented electrical steel sheet due to shearing can be reduced. As a result, iron cores having a low iron loss can be obtained and thus, for example, large transformers having high energy efficiency can be produced. 

1. A grain oriented electrical steel sheet comprising, by mass %, 0.005% or less of C, 1.0% to 8.0% of Si, and 0.005% to 1.0% of Mn; one or more selected from the group consisting of Nb, Ta, V, and Zr such that a total content thereof is 10 to 50 ppm; and the balance being Fe and unavoidable impurities, wherein at least 10% of the content of Nb, Ta, V, and Zr is in the form of precipitates; the precipitates having an average diameter (equivalent circle diameter) of 0.02 to 3 μm; and secondary recrystallized grains of the steel sheet have an average grain size of 5 mm or more.
 2. The grain oriented electrical steel sheet according to claim 1, further comprising at least one selected from the group consisting of, by mass %, 0.010% to 1.50% of Ni, 0.01% to 0.50% of Cr, 0.01% to 0.50% of Cu, 0.005% to 0.50% of P, 0.005% to 0.50% of Sn, 0.005% to 0.50% of Sb, 0.005% to 0.50% of Bi, and 0.005% to 0.100% of Mo.
 3. The grain oriented electrical steel sheet according to claim 1, wherein a groove is formed in a surface of the steel sheet, the groove having a shape of a solid line or broken lines, a width of 50 to 1,000 μm, and a depth of 10 to 50 μm, and extending at an angle of 15° or less with respect to a direction perpendicular to a rolling direction of the steel sheet.
 4. A method for producing an iron core, comprising shearing the grain oriented electrical steel sheet according to claim 1 to provide sheets and subsequently stacking the sheets without subjecting the sheets to stress relief annealing.
 5. The grain oriented electrical steel sheet according to claim 2, wherein a groove is formed in a surface of the steel sheet, the groove having a shape of a solid line or broken lines, a width of 50 to 1,000 μm, and a depth of 10 to 50 μm, and extending at an angle of 15° or less with respect to a direction perpendicular to a rolling direction of the steel sheet.
 6. A method for producing an iron core, comprising shearing the grain oriented electrical steel sheet according to claim 2 to provide sheets and subsequently stacking the sheets without subjecting the sheets to stress relief annealing.
 7. A method for producing an iron core, comprising shearing the grain oriented electrical steel sheet according to claim 3 to provide sheets and subsequently stacking the sheets without subjecting the sheets to stress relief annealing.
 8. A method for producing an iron core, comprising shearing the grain oriented electrical steel sheet according to claim 5 to provide sheets and subsequently stacking the sheets without subjecting the sheets to stress relief annealing. 